Even when spirits are involved, there are different classes of spirits. I'd understand their efficacy when fighting against anyone else, but a 9000lvl Fenrir or 9999lvl warrior, I'd expect those spirits to use their life force just to deflect an attack. And it'd probably take more than one spirit to do it. And the more spirits are gone, the more the user has to summon or create, eating away at his own mana reserves.
It'd be like a Stamina battle, but one lost in advance because again: He's lvl 7000 vs one lvl 9000 and two lvl 9000. Fenrir alone should have dealt with it easy.